Galton's method in Hereditary Genius was to count and assess the eminent relatives of eminent men. He found that the number of eminent relatives was greater with closer degree of kinship. This work is considered the first example of historiometry, an analytical study of historical human progress. The work is controversial and has been criticised for several reasons. Galton then departed from Gauss in a way that became crucial to the history of the 20th century AD. The bell-shaped curve was not random,Genius Seasons 1-2 DVD he concluded. The differences between the average and the upper end were due to a non-random factor, "natural ability", which he defined as "those qualities of intellect and disposition,Genius Seasons 1-2 which urge and qualify men to perform acts that lead to reputation…a nature which, when left to itself, will, urged by an inherent stimulus, climb the path that leads to eminence."The apparent randomness of the scores was due to the randomness of this natural ability in the population as a whole, in theory.